Kołakowski discusses the origins, philosophical roots, golden age and breakdown of Marxism, and the various schools of Marxist philosophy.

He describes Marxism as "the greatest fantasy of the twentieth century", a dream of a perfect society which became a foundation for "a monstrous edifice of lies, exploitation and oppression."

[3] His first volume discusses the intellectual background of Marxism, examining the contributions of Plotinus, Johannes Scotus Eriugena, Meister Eckhart, Nicholas of Cusa, Jakob Böhme, Angelus Silesius, Jean-Jacques Rousseau, David Hume, Immanuel Kant, Johann Gottlieb Fichte, Ludwig Feuerbach, Georg Wilhelm Friedrich Hegel, and Moses Hess, as well as an analysis of the work of Karl Marx and Friedrich Engels.

"[5] The second volume includes a discussion of the Second International and figures such as Paul Lafargue, Eduard Bernstein, Karl Kautsky, Georges Sorel, Georgi Plekhanov, Jean Jaurès, Jan Wacław Machajski, Vladimir Lenin, Rosa Luxemburg, and Rudolf Hilferding; it reviews Hilferding's debate about the theory of value with the economist Eugen Böhm von Bawerk.

[6] The third volume deals with Marxist thinkers such as Leon Trotsky, Antonio Gramsci, Lukács, Joseph Stalin, Karl Korsch, Lucien Goldmann, Herbert Marcuse, Jürgen Habermas, and Ernst Bloch, as well as the Frankfurt School and critical theory.

[7] In the preface added to the 2005 edition, Kołakowski attributed the demise of communism in Europe partly to the collapse of Marxism as an ideology.

"[8] According to Kołakowski, Main Currents of Marxism was written in Polish between 1968 and 1976, at a time when it was impossible to publish the work in Communist-ruled Poland.

[10] Main Currents of Marxism has been praised by authors such as the philosophers A. J. Ayer, Roger Scruton, and John Gray, the conservative commentator William F. Buckley Jr., and the political scientists Charles R. Kesler and David McLellan.

[13] Buckley and Kesler called Main Currents of Marxism "excellent" and credited Kołakowski with demonstrating "the connection between Marxist theory and Stalinist reality".

[16] Smith wrote that while Main Currents of Marxism is "written by a deeply disabused Polish ex-Marxist intellectual", it is "an invaluable history across an extended range.

"[17] The philosopher Richard Rorty wrote that eastern and central Europeans who have read Kołakowski suspect that he tells you "pretty much all you will ever need to know about Marx and Marxism–Leninism.

"[18] Critical views of the book include those of the political theorist Paul Thomas and the philosophers M. W. Jackson and Jon Stewart.

[19] Thomas argued that Kołakowski, motivated by the wish to connect Marx to "his self-appointed disciples", wrongly interprets Marxism as "radical anthropocentrism, a secularization of the (real) religious absolute, a formula for human self-perfectibility, and the self-deification of humankind.

He credited Kołakowski with summarizing the strengths and weaknesses of Marxism, and praised his discussions of historical materialism, Engels's Dialectics of Nature, Kautsky, Plekhanov, Leninism, Trotsky, Trotskyism, Lukács, Marcuse, and Althusser.

He criticized Kołakowski for ignoring and neglecting important elements in Marxian economics, such as Paul Sweezy's revival of the work of Ladislaus von Bortkiewicz in The Theory of Capitalist Development (1942).

[23] Jay described the book as "extraordinarily valuable" and "powerfully written", "an awesome achievement of synthetic scholarship and critical analysis", and a "monument of committed learning".

He praised Kołakowski's account of the "origins of dialectic", and his discussion of the philosophical aspects of Marx's theory, though considering them relatively unoriginal.

He wrote that while Kołakowski's work contained occasional factual errors or dubious interpretations, there were remarkably few of them considering its length.

He praised Kołakowski's discussion of Polish Marxists, Gramsci, Lukács, the Frankfurt School, Bernstein, historical materialism, Hess's influence on Marxism, the recognition of individuality in "Marx's social ideal", the failure of attempts to resolve contradictions between the first and third volumes of Das Kapital, the concept of exploitation in Marx, and Jaurès and Lafargue.

However, he criticized Kołakowski for his treatment of some Marxists, and for ignoring the sociologist Lewis Samuel Feuer's "research on the influence of American utopian socialist colonies on Marx."

He credited Kołakowski with showing "the essential continuity of development" of Marx's thought up to its culmination in Das Kapital, and considered his chapters on Lukács, Bloch, Marcuse, the Frankfurt School, and Mao Zedong especially valuable.

[29] Joravsky saw the book as a continuation of arguments over Marxism that Kołakowski had previously engaged in while a member of the Polish United Workers' Party.

He found much of Main Currents of Marxism dull, and considered Kołakowski's account of Marx's place in the history of philosophy tendentious.

He gave a more favorable assessment of his discussions of Austromarxism, Polish Marxism, and Habermas, but was dissatisfied with his treatment of the Communist movement as a whole, accusing him being preoccupied with Russia and neglecting other countries.